Based on the success of vaccines for the treatment and prevention of infectious diseases, there have been attempts over the years to develop cancer vaccines. Early attempts at cancer vaccines were based on immunising the cancer patient with killed tumour cells together with adjuvant.
Further development of cancer vaccines has focused on directing the immune response to identified tumour antigens present on the surface of tumour cells. The advent of monoclonal antibodies enabled the identification of so-called “tumour-specific” antigens. Typically, however, tumour antigens not tumour-specific, but are over-expressed on tumour cells and rarely expressed, or under-expressed, on normal cells.
A well-characterised tumour antigen is “polymorphic epithelial cell mucin” (PEM), the product of the MUC1 gene. PEM is over-expressed and aberrantly glycosylated in many carcinomas (breast, pancreas, ovary, lung, urinary bladder, prostate, and endometrium) thus resulting in cancer cells expressing antigenically-distinct PEM molecules on their surface. Both humoral and cellular immune responses specific to PEM have been detected in cancer patients. These properties make PEM particularly well-suited to act as a target molecule for cancer immunotherapy. In addition, the over-expression of mucin on tumour cells suggests that the mucin may be important for the maintenance of the tumour. Such over-expression of mucin would alleviate potential problems of tumour cells escaping immunotherapy by down-regulating the MUC1 gene. U.S. Pat. Nos. 4,963,484 & U.S. Pat. No. 5,053,489 and international patent application WO88/05054 describe the purification of human mucin, the isolation of monoclonal antibodies thereto, the cloning of cDNAs coding for human mucin, and the use of these for diagnostic and therapeutic applications.
Zhang et al. [Cancer. Res. (1996) 56:3315–3319] disclose the preparation of several MUC1 peptide vaccines and compares their immunogenicity. Vaccines comprising adjuvant and a MUC1 peptide covalently linked to the protein carrier KLH achieved greatest immunogenicity. The authors concluded that MUC1 peptides of 30 or more amino acids are better immunogens than MUC1 peptides of 20 amino acids.
The MUC1 gene product is a high molecular weight (MW>200 kDa) transmembrane glycoprotein expressed on the apical surface of many simple epithelial cells. It has a relatively large extracellular domain varying from 1000 to 2200 amino acids and a cytoplasmic tail of 69 amino acids. The extracellular domain consists mainly of tandem repeats of the 20 amino acid sequence PDTRPAPGSTAPPAHGVTSA, set forth as SEQ ID NO:1 [for a review, see Apostolopoulos & McKenzie (1994) Crit. Rev. Immunol., 14: 293–309]. Variability in the number of repeats accounts for the variability in the size of the extracellular domain. Each repeat contains five potential O-linked glycosylation sites and two or possibly three of these sites are believed to be utilised.
The aberrant glycosylation of PEM observed in a variety of cancers is due to alterations in the activity of glycosyltransferases with some transferases being inactive and the activity of others being increased. Abnormal glycosylation of PEM in cancer cells leads to three types of cancer-associated epitopes: (a) peptides, from the core protein, that are exposed following under-glycosylation; (b) “new” carbohydrates resulting from deficient and aberrant glycosylation; and (c) “new” glycopeptides resulting from deficient and aberrant glycosylation.
Many different approaches have been considered as potential cancer vaccines aimed at inducing a beneficial immune response directed towards PEM-expressing tumour cells. These include: cells expressing mucins: mucin purified from cancer cells; mucin core protein produced as a recombinant protein; different mucin glycoproteins produced by transfected cells; peptide or glycopeptide based on the tandem repeat; recombinant animal viruses expressing portions of the MUC1 cDNA gene product; synthetic carbohydrates based on short aberrant chains present on mucins on cancer cells; naked MUC1 DNA; and anti-MUC1 antibodies.
Of these, anti-MUC1 antibodies, synthetic carbohydrates, recombinant animal viruses, and peptides based on the tandem repeat have been developed sufficiently to reach evaluation in clinical trials. However, one significant problem with all of the above approaches is that a poor and/or inadequate immune response towards mucin-expressing tumour cells is observed.
It is an object of the present invention to alleviate this problem by providing an effective means for presenting a peptide of a tumour-associated mucin.